The overall objective of this work is to detail the mechanism of selective IgG transport across the small intestine of the neonatal rat. The system represents one of the best documented examples of receptor-mediated transport of a specific protein across cells. Studies which will be continued during the coming year include (a) Assessment of antigen transfer across the neonatal intestine, (b) Isolation of the membrane receptor for IgG in the intestine (c) Measurement of the effect of IgG concentration on rate of pinocytosis, and (d) A comparison of the intestinal transport system with that in the fetal yolksac.